High hardness prealloyed steel powder, such as tool steel grade of powders, can either be used alone or admixed with other powder metal compositions in the powder-metallurgy production of various articles of manufacture. Tool steels contain elements such as chromium, vanadium, molybdenum and tungsten which combine with carbon to form various carbides such as M6C, MC, M3C, M7C3, M23C6. These carbides are very hard and contribute to the wear resistance of tool steels.
The use of powder metal processing permits particles to be formed from fully alloyed molten metal, such that each particle possesses the fully alloyed chemical composition of the molten batch of metal. The powder metal process also permits rapid solidification of the molten metal into the small particles which eliminates macro segregation normally associated with ingot casting. In the case of highly alloyed steels, such as tool steel, a uniform distribution of carbides can be developed within each particle, making for a very hard and wear resistant powder material.
It is common to create the powder through atomization. In the case of tool steels and other alloys containing high levels of chromium, vanadium and/or molybdenum which are highly prone to oxidation, gas atomization is often used, wherein a stream of the molten alloy is poured through a nozzle into a protective chamber and impacted by a flow of high-pressure inert gas such as nitrogen which disperses the molten metal stream into droplets. The inert gas protects the alloying elements from oxidizing during atomization and the gas-atomized powder has a characteristic smooth, rounded shape.
Water atomization is also commonly used to produce powder metal. It is similar to gas atomization, except that high-pressure water is used in place of nitrogen gas as the atomizing fluid. Water can be a more effective quenching medium, so that the solidification rates can be higher as compared to conventional gas atomization. Water-atomized particles typically have a more irregular shape which can be more desirable during subsequent compaction of the powder to achieve a greater green strength of powder metal compacts. However, in the case of tool steels and other steels containing high levels of chromium, vanadium and/or molybdenum, the use of water as the atomizing fluid would cause the alloying elements to oxidize during atomization and tie these alloying elements up making them unavailable for reaction with carbon to form carbides. Consequently, if water atomization were employed, it may need to be followed up by a separate oxide reduction and/or annealing cycle, where the powder is heated and held at an elevated temperature for a lengthy period of time (on the order of several hours or days) and in the presence of a reducing agent such as powdered graphite, or other source of carbon or other reducing agent or by another reducing process. The carbon of the graphite would combine with the oxygen to free up the alloying elements so that they would be available for carbide formation during the subsequent sintering and tempering stages following consolidation of the powder into green compacts. It will be appreciated that the requirement for the extra annealing/reducing step and the addition of graphite powder adds cost and complexity to the formation of high alloy powders via the water atomization process.